What is a WBGT forecast?
A WBGT forecast predicts heat stress, not temperature. Instead of telling you it will be 96 °F on Thursday, it tells you how hard Thursday afternoon will be on a body that is working, and therefore whether a practice, a shift or a training evolution should be moved, shortened, or left alone.
That is a different question from "how hot will it be," and it needs different inputs.
Why an ordinary forecast can't give you one
Wet-bulb globe temperature combines four things: air temperature, humidity, wind, and direct sunlight. A standard forecast hands you the first two prominently, the third if you look, and the fourth almost never.
The sun term is the one that matters most and is hardest to come by. On a clear afternoon, solar radiation is what separates a reading that is merely uncomfortable from one that stops play. It is also why you cannot estimate WBGT by taking the forecast high and subtracting a few degrees: the offset between air temperature and WBGT is not a constant. It moves with cloud cover, wind and sun angle, hour by hour.
Two days can share a forecast high of 94 °F and sit a full tier apart on WBGT.
How this forecast is computed
We run the Liljegren (2008) energy-balance model, the reference method behind research-grade WBGT, over hourly forecast data from Open-Meteo's model blend. Every input the model needs is forecast, not inferred: temperature, humidity, wind speed, and solar radiation, with sun angle computed for the location and hour.
Two numbers come out of it, and both are published:
- In the sun, the standard WBGT, which is the number policies are written against.
- In full shade, computed under a stated convention, because the same hour can be two different decisions depending on where people actually stand.
The full derivation, including every constant and assumption, is in the methodology. We think a forecast you cannot inspect is a forecast you should not act on.
How far ahead it is useful
The app carries WBGT out to 48 hours, which is the window where the number is precise enough to schedule against.
Be realistic about what happens beyond that. A WBGT forecast inherits the uncertainty of the weather forecast underneath it, and the solar term degrades faster than the temperature term, because cloud cover is harder to predict than heat. Two days out, treat it as a planning signal — the shape of the week, whether Thursday looks worse than Wednesday — rather than a number to write into a schedule.
What a forecast cannot do
This is the part worth being blunt about.
A forecast is modeled, not measured. It describes a grid cell, not the particular patch of turf your team is standing on, and it cannot see the things that make one field hotter than another: a stadium bowl that traps air, a black track, a windbreak of trees.
Where a policy calls for an on-site instrument reading to govern activity — which is how most state athletic association heat rules and workplace heat plans are written — that reading governs. Nothing modeled replaces it, ours included.
What a forecast is genuinely good for is everything that happens before anyone is standing on the field with a meter: deciding on Tuesday whether Thursday's practice should start at 7 a.m., whether to plan for full pads, whether to schedule the heaviest task for the morning. By the time an on-site reading exists, most of the decisions it informs have already been made.
Checking the number
A live WBGT is a model, so the honest thing is to show how it performs rather than assert that it is good.
We publish a live comparison of our WBGT against the National Weather Service's own published WBGT, recomputed continuously and shown with the disagreements intact. Read the caveat there too: part of any gap is the two systems differing about clouds and sunlight at that hour, not about the heat-stress calculation. It is a comparison against another model, not against a calibrated instrument, and we say so.
Getting a forecast for your location
Search any place on the map for the live reading and the 48-hour forecast in sun and shade, or open your city page for the same thing with local climate context. Coaches, crews and instructors can start from the athletics, outdoor work or tactical views, which pair the forecast with the guidance written for that setting and, for schools, a link to the state association's own policy document.
No account, and nothing safety-related behind a paywall.